The present invention relates to a method and/or architecture for wide hand amplifiers generally and, more particularly, to a method and/or architecture for laser driver amplifiers that employ a method of shaping or equalizing a high data rate output signal waveform that may be used in fiber optic transmitter applications.
The need for broadband integrated circuits (ICs) that incorporate peaking control is becoming a necessity rather than a luxury as the data rates and traffic of fiber optic systems increase. Peaking control in wideband integrated circuits is useful for adjusting gain and amplitude peaking in the frequency domain. Fiber optic data links require adjusting signal overshoot in the time domain for shaping the waveform of transmit and receive signals to achieve lower bit error rates. The need for adjustment of signal overshoot becomes more imperative for 10 gigabit per second fiber optic applications that require high volume-high IC yield with low tolerance to semiconductor process variations. Such applications include the emerging 10 gigabit Ethernet datacom systems.
A broadband IC which can modulate lasers or vertical cavity surface emitting lasers (VCSELs) at data rates up to 10 Gb/s and maintain low bit error rates is coveted by engineers building high speed Ethernet systems. In practice, laser driver ICs require pre-emphasis circuits or some type of control circuit which can compensate for the distortion introduced by the nonlinear laser or VCSEL. The signal passing through a linear laser driver modulator operating at 10 Gb/s can become distorted in the process of converting from electrical to optical energy as the laser diode or VCSEL is modulated by the linear electrical driver circuit. When conversion distortion occurs, it is desirable to drive the laser or VCSEL with a pre-distorted signal which compensates for the distortion produced by the nonlinear behavior of the VCSEL or laser. The pre-distortion may be implemented by a pre-emphasis or peaking function which superimposes a weighted peaking signal on the original signal to speed up the rise and falling edges of the original data waveform. The superimposed peaking signal enhances the data transition rise and fall times as well as reshapes the signal for low bit error rates (BER), inter-symbol interference (ISI), and maximum eye pattern opening.
Several conventional approaches for employing such pre-distortion comprise [1] dynamic current source switching (e.g., Rainer H. Derksen, Novel Switched Current Source for Increasing Output Signal Edge Steepness of Current Switches Without Generating Large Overshoot, IEEE JSSC, vol. 30, no. 5, May 1995) and [2] pre-emphasis (digital peaking) (e.g., Ramin Farjad-Rad, et. al., A 0.4-um CMOS 10-Gb/s 4-PAM Pre-Emphasis Serial Link Transmitter, IEEE JSSC, vol. 34, no. 5, May 1999), each of which is incorporated by reference in its entirety. Additionally, analog peaking techniques may be employed. The first two techniques are common-types of approaches which have had practical implementations at low data rates (i.e., 2.5 Gb/s and below). However, the implementations of these techniques at higher data rates is challenging. The effectiveness of these techniques can be marginal at 10 Gb/s and higher due to the quality of the raw data signal which provides a clock or trigger for the technique.
The analog peaking technique approach does not rely on using the raw data waveform as a clock or synchronizing signal for the technique to be effective and is therefore more suitable for higher data rate applications. Due to the recent availability of long and short wave VCSEL technology for 10 Gb/s, an effective device for employing pre-emphasis or peaking control is desired. Additionally, a device which is amenable to high volume-high yield manufacturing for 10 Gb/s VCSEL driver applications is needed.
The present invention concerns an apparatus comprising a first amplifier, a second amplifier and a control circuit. The first amplifier may be configured to present a first amplified output signal in response to an input signal. The second amplifier may be configured to present a second amplified output signal to provide a shaped signal peaking response in response to the input signal. The first and second amplified output signals are generally combined. The control circuit may be configured to control a ratio between the first amplified output signal and the second amplified output signal. The ratio controls an amount of the peaking response.
The objects, features and advantages of the present invention include providing a method and/or architecture for implementing laser driver amplifiers that may (i) provide shaping or equalizing of a high data rate output signal waveform; (ii) be used in fiber optic transmitter applications; (iii) employ a switched architecture for switching between a regenerative peaking amplifier and a conventional amplifier; (iv) implement a switching circuit that may allow variable weighted employment of the peaking amplifier; (v) provide AC coupling of the peaking amplifier with a speedup capacitor and resistor which may allow a degree of freedom to set up a decay time constant; (vi) implement variability in AC coupling, and/or (vii) implement a tap point that may optimize the time superposition of the peak and unpeaked signals at the output.